1 / 17

Continuation Passing Style

Continuation Passing Style. Roadmap. Continuation Passing Style CPS and tail call CPS transformation. Continuation. Can we use functions to represent the control flow in a program? Yes , using the concept of a continuation . . Continuation.

nedra
Download Presentation

Continuation Passing Style

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Continuation Passing Style

  2. Roadmap • Continuation Passing Style • CPS and tail call • CPS transformation

  3. Continuation • Can we use functions to represent the control flow in a program? • Yes, using the concept of a continuation.

  4. Continuation • Augment each procedure with an additional argument • The outer procedure “returns” no result, but pass the result to the argument • This function argument, receiving the result, will be called the continuation.

  5. CPS Writing procedures so that theytake a continuation to which they pass on the computation result, and they return no result, is called continuation passing style (CPS)

  6. CPS • CPS provides a programming technique for all forms of “non-local” control flow: • non-local jumps • exceptions • etc

  7. CPS • CPS turns all non-tail calls into tail calls. • CPS also acts as a compilation technique to implement non-local control flow • Also acts as a formalization of non-local control flow in denotationalsemantics

  8. Roadmap • Continuation Passing Style • CPS and tail call • CPS transformation

  9. Ex1. Simple reporting fun report x = (print (Int.toString(x)); print "\n") fun plusk a b k = k ( a + b ) val _ = plusk 20 22 report

  10. Ex2. Factorial fun fac n = if n = 0 then 1 else n * (fac (n - 1)) fun fack n = let fun fack_helper n k = case n of 0 => k 1 | _ => fack_helper (n - 1) (fn r => k (r * n)) in fack_helper n (fn x => x) end

  11. Ex3. Length fun length xs = case xs of [] => 0 | _::xs' => 1 + length xs' fun lengthkxs k = case xs of [] => k 0 | _::xs' => lengthkxs' (fn r => k (1 + r))

  12. Roadmap • Continuation Passing Style • CPS and tail call • CPS transformation

  13. CPS transformation 1. Add a continuation argument to any function definition: fun f args = e  fun f args k = [e]k Here we use [e]k to mean the transformation will continue in e with continuation k

  14. CPS transformation 2. Expression with no available function callsin tail position should be passed to a continuation instead of returned return a  k a

  15. CPS transformation 3. Augment every function call in tail position by passing it the current continuation return f argsf args k

  16. CPS transformation 4. Now, each function call not in tail position needs to be built into a new continuation, containing the old continuation and the remainder of the operation that still needs to be performed. op (f args)  f args (fnr => [op r]k) [More examples see the code file]

  17. Conclusion • Continuation Passing Style • CPS and tail call • CPS transformation

More Related